DEVIATION AND ITS CAUSES 19 



and in turn the hole. Thus long barrels are often faulty 

 for want of strength and undue pressure on the crown. 

 There appears to be much in favor of bigger holes and 

 reduction not proceeding beyond l^i in. at 2,000 ft. Weak 

 barrels may cause screw deflection. The crown often 

 returns to its original direction after deflection has occurred 

 in some West Australian borings. With big rod reductions 

 the play cannot be entirely eliminated at the step joint. 



k. Static Electricity and Magnetism of Rods. — This effect 

 due to frictional abrasion is often very pronounced and 

 can be demonstrated by means of a poker of soft iron, a 

 hammer and compass. It must, if of definite persistent 

 polarity, tend to deviate the rods toward the pole sought.^ 

 Magnetism will tend to arise also from brushing with casing 

 and the strata if heavily iron borne as in the basic igneous 

 rocks. Some further notion of the causes of borehole 

 deviation may be obtained by considering the eventualities 

 inherent in all boreholes, as yet beyond human control, 

 as are evidenced in any attempt to fix the dip of strata abso- 

 lutely from observation on a given core. 



Only approximately can we obtain the dip angle of strata 

 bored through by considering the core features alone. 

 This is very simple but the estimating of the direction of the 

 dip and thence the strike of the beds in such a case cannot 

 be done without some form of stratameter which gives 

 the dip and strike accurately from the data presented. 

 The objection here is that the observation is too local and 

 the data too scanty. We have to assume that the core 

 yielding the data has been accurately gripped by the core 

 catcher. Thus in the surface check on the core no account 

 has been taken of the turn of the rods on tearing off the core 

 previous to extraction. An American method of partially 

 avoiding this is to score a continuous line down the rods 

 after tightening with special joints and then check the dip 

 shown against this line of known azimuth. Now the 

 longer the line of rods and tools the less can they be regarded 



^ Jennings, J., Jour. S. African Assoc. Eng., Vol. 12, p. 7, 1906; Cooke, 

 L. H., Trans. Inst. Min. and Met., Seventeenth Session, p. 126, 1907. 



